Status quo on recycling of waste crystalline silicon for photovoltaic modules and its implications for China’s photovoltaic industry

Abstract

As a clean and efficient renewable energy source, solar energy has been rapidly applied worldwide. The growth rate of China’s installed capacity ranks first in the world. However, the life span of photovoltaic (PV) modules is 25 to 30 years, and the rapid development of installed capacity indicates that a large number of PV modules will be decommissioned in the future. Therefore, the ongoing treatment of the scrapped PV waste cells in the near future requires urgent plans and countermeasures. Proper recycling and disposal of decommissioned PV modules is a practical requirement for the sustainable development of the country and industry. Crystalline silicon (c-Si) solar cells currently occupy 85%–90% of the market share, and some scholars have begun to seek the utilization pathways of the waste Si in and outside the PV industry. In this paper, the research status of the separation and recycling process of crystalline Si PV modules is reviewed, and the recycling ways of crystalline silicon are particularly focused on. In addition, the current bottlenecks in the PV recycling industry in China are analyzed and some suggestions on the sustainable development of the PV industry are proposed.

Graphical abstract

Keywords

waste photovoltaic (PV) modules / crystalline silicon (c-Si) battery / separation and recovery / sustainable development

1 Background

With the world’s continuous growth of population and economy, traditional fossil energy is consumed in large quantities. Continuously developing and utilizing non-renewable energy will lead to energy shortage, restrict economic growth, and cause global warming due to large carbon emissions [1]. Under the pressure of energy shortage and environmental pollution, countries worldwide have turned their attention to renewable energy and competed to promote development. In wind energy, nuclear energy, solar energy, and many other environmentally friendly renewable energies, the solar photovoltaic (PV) system has become one of the fastest-growing clean energy because of its relatively low cost. Solar energy resources are inexhaustible, widely distributed, safe, and pollution-free, and can be converted into electricity using the PV power generation technology, which has a very broad application scenario [2].

China, one of the largest energy consumption countries in the world, has a vast territory, rich solar energy resources, and a huge PV technology application market potential [3]. In September 2007, the Chinese government issued the “China Renewable Energy Medium- and Long-Term Development Plan”, which proposed to include wind energy, solar energy, etc., into the national renewable energy development goals. In February 2012, the Ministry of Industry and Information Technology of China officially issued the “12th Five-Year Development Plan for the Solar Photovoltaic Industry” in the context of policy encouragement and energy transformation. China has become a country with the world’s fastest-growing PV installed capacity (Fig.1) [4,5]. However, the life of PV modules is generally 20 to 30 years, and the scale of PV installed capacity continues to expand, which also indicates that many PV modules will be at the end of their life in the near future. By 2030, 1 million tons of PV modules will be scrapped globally [6]. By 2050, China’s PV panel scrap volume will be the first in the world, about 33 million tons [4]. In October 2021, the “Action Plan for Carbon Dioxide Peaking Before 2030” issued by the State Council of China pointed out that it is necessary to promote the recycling of waste from emerging industries such as decommissioned power batteries, PV modules, and wind turbine blades. It can be seen that the resource recovery of waste PV modules has realistic needs for China’s national strategic development. Doing a good job of recycling programs before the first batch of PV panels is retired will be related to the healthy development of the entire industry in the future.

Due to the harmful heavy metal elements such as cadmium (Cd) and lead (Pb) contained in PV waste, improper treatment methods may cause these substances to pollute water or soil, thus posing a threat to animal, plant, and human health [9]. Therefore, the European Union amended the Waste Electrical and Electronic Equipment (WEEE) regulation in 2012 to classify decommissioned PV modules as waste electronic equipment. Subsequently, it introduced PV waste management regulations to make requirements for reasonably disposing of waste PV modules [10]. At the same time, PV waste is also a potential resource pool, which contains a large number of common materials such as aluminum (Al) and glass, as well as high-value materials such as copper (Cu), silver (Ag), and silicon (Si) [11]. For example, PV Ag paste is one of the core auxiliary materials for preparing PV cells. A 72-piece module of PV cells requires 7.9 g of Ag paste, and an annual production needs about 160 tons of Ag paste for a medium-sized PV company. With the recent rise in Ag prices, purchasing new silver will cost $36000 ($ denotes USD) monthly [12]. Therefore, the scientific and reasonable recycling of PV waste not only reduces the potential environmental risks but also avoids the loss of valuable materials and reduces the demand for new material production, which is of great significance for the sustainable development of the PV industry.

Existing solar cells can be mainly divided into crystalline silicon (c-Si) cells and thin film cells. Because of their low production cost and high-power generation efficiency, c-Si cells have dominated the PV market with a share of 85%–90% in the past decades (Fig.2) [8]. There are many discussions on recovering and purifying Ag, Al, Cu, and other elements in c-Si cells. However, the recycling and utilization of Si have not been discussed in detail. As a semiconductor material, high-purity monocrystalline Si is the primary raw material for preparing chips and the mainstream material for solar cells, which plays a significant role in national development. Si is highly abundant in nature, but its mining and processing are energy-intensive [13]. Recycling Si from PV waste can reduce the production cost of solar-grade Si [14]. The recovered high-purity Si can replace the original metallurgical grade Si (MG-Si) while reducing energy consumption and greenhouse gas emissions by more than 60% [14]. If the Si purity is less than 98%, it can be used in aerospace, electronic appliances, energy, agriculture, environmental protection, and other fields. At present, the recovery cost of PV modules maintains high while the economic efficiency of its recycling treatment stays low, which hinders the benign development of the PV industry. Therefore, it is particularly critical to innovate the recycling process, explore the way of reproduction, and form an industrial closed loop. Some scholars have begun to study the Si recovery and recycling process to explore more efficient and energy-saving Si recovery ways.

China’s PV industry has yet to face large-scale decommissioning and build a complete recycling industrial system. The PV recycling industry in Europe, the United States, and other countries developed earlier than in China, with more mature recycling technology systems and policies [15]. To cope with the massive demand for PV module recycling and prevent the loss of valuable materials and environmental pollution, China’s recycling technology and industrial system need to be further developed [16]. Based on the analysis of relevant literature in recent decades, this paper discusses the recycling technology of existing PV modules, summarizes the recycling ways of Si, and analyzes the predicament of the China PV recycling industry at the present stage, aiming to provide references and suggestions for the development of the Si waste solar cell recycling technology and its industrial utilization.

2 Method of separation and recovery

2.1 Structure of PV modules

Currently, the most widely used types of solar cells worldwide are crystalline Si cells, including monocrystalline Si and polysilicon, which have similar structures. Fig.3 shows the overall structure of these two types of PV modules, consisting of an external metal frame, a junction box, cables, and an internal battery module. The battery module comprises a glass cover plate, an encapsulating agent, a battery sheet, and a backplane. Among them, the Si cell is the core component of photoelectric conversion. It is composed of an Ag electrode, an anti-reflection coating (ARC), n-type Si and p-Si, an Al back electrode, an Al−Si alloy, and an Al electrode [17]. After being separated from the transmission modules, the external frame components can be recovered by metallurgical means. Due to the sandwich structure of the internal PV modules, it is necessary to go through the separation and extraction process before and after to truly realize the recycling and reuse of scrapped PV modules. The primary recycling process for end-of-life PV modules includes removing the outer frame and the junction box, separating bonding materials, and extracting and purifying the final target element [18]. Fig.4 shows a feasible recycling technique for end-of-life c-Si solar PV panels. The first step is to delaminate the components, mainly by thermal, mechanical, and chemical delamination, while the second step is to recover the Si cell or metals. Some typical examples of using various processing methods, with details illustrated in the following sections, are summarized in Tab.1.

2.2 Physical processing

Physical treatment means the separation of components through external force (such as crushing/cutting [22], high-voltage fragmentation/pulse [20,21], and electrostatic separation [35]). This is often the first step after removing the junction box and frame by sifting particles of different sizes. In the process of mechanical recycling, some researchers attempt to obtain undamaged recycled materials, while in the others materials are completely crushed. Akimoto et al. [20] applied the separation of high-voltage pulse and heavy media to the selective separation and recovery of PV panels (Fig.5(a)). The separation efficiency of the material is improved by two-pulse processing. The first pulse was found to maintain the intact shape of the Si wafer and separate the backplane at 110 kV. The second pulse can complete the separation of glass and encapsulant under all conditions, but the high voltage leads to the failure of the sealant. In the end, they concluded that 90 kV and 250 pulses are the best conditions for achieving mutual separation without damaging the materials in the sandwich structure. Li et al. [23] also attempted to recycle undamaged Si wafer. They successfully striped EVA by laser irradiation. Song et al. [21] also used high-voltage fragmentation with a higher voltage of 160 kV and a pulse of 300 times, and completely broke the PV modules. The resulting particle size of the product was smaller, and selective fragmented products (e.g., Cu, Al, Pb, Ag, and Sn) are concentrated on the fractions under 1 mm. Although they separated the components, the materials recovered by the density grading were still mixtures and could not be purified as a single component. Granata et al. [22] separated and recycled PV modules by cutting and hammering. Then, they heat-treated the fragments to remove the encapsulation agent. Finally, they screened and recovered the treated products with different particle sizes. Although this method could not obtain a single pure component, it was found that the metal elements were more concentrated in the component with a diameter < 0.08 mm (Fig.5(b)), improving the subsequent metal extraction efficiency.

Physical processing can efficiently handle PV modules on a large scale, and the mechanized and automated recycling process will not cause environmental pollution. However, the resulting products often exist in mixtures, and subsequent separation and purification technology needs to be developed to recover a single component.

2.3 Pyrolysis

The Si battery is bonded with the backplane and the glass cover plate through organic packaging. The most widely used encapsulation agent is EVA. Doi et al. [36] studied the influence of different organic solvents on EVA removal. The non-destructive Si wafers could be separated and recovered by treating Si cells with trichloroethylene at 80 °C for 10 d. Weakening the viscosity of EVA through heat treatment or breaking it down is a more effective alternative to chemical processes using expensive and toxic agents [31].

When the heating temperature is low, EVA will be softened [37]. Li et al. [23] used the method of mechanical stripping after laser irradiation to recover the EVA layer on the back of c-Si cell. The viscosity of EVA decreased after being heated. However, this technology has high requirements for equipment, and EVA does not have recycling value. Other scholars have studied the effect of higher heat treatment temperatures on the thermal decomposition of EVA. Tammaro et al. [24] found that the packaging polymer could be completely removed by pyrolyzing the waste PV panels at 600 °C for 30 min. Dias et al. [25] pyrolyzed waste PV panels at 500 °C for 30 min, and observed that the EVA removal rate reached 99%, effectively reducing the energy consumption. Wang et al. [27] adopted a two-step heating method, heating at 330 °C for 30 min to remove the backplane and then heating at 400 °C for two hours to remove EVA. Still, the effective reduction of the energy consumption of pyrolysis is the focus of heat treatment research.

Heat treatment can efficiently remove EVA, but the resulting battery is often broken to varying degrees, and it is related to whether the heating equipment can heat the material uniformly [38]. At the same time, it is difficult to ensure the purity of the battery after pyrolysis. EVA is oxidized and form a small amount of carbon remains on the surface [39]. The pyrolysis of EVA also produces olefins, alkanes, a small number of aromatic compounds and alcohols, and some harmful metals entering the gas phase. Therefore, it is necessary to purify the pyrolysis gas further to prevent it from polluting the environment [17–40].

2.4 Chemical treatment

Physical and heat treatment can separate each component, but the resulting products are mostly mixtures. For the recovery of valuable metal elements and Si elements in PV modules, it is necessary to extract and refine the mixtures by chemical means (Fig.6).

To recover high-purity Si wafers, Ag electrodes, Al electrodes, and anti-reflection layer (SiN_x) need to be removed from their surface. The removal of SiN_x usually requires HF, Br₂ mixtures, or H₃PO₄. Kang et al. [29] immersed the EVA-removed battery in the mixture of HF, HNO₃, H₂SO₄, and CH₃COOH at room temperature for stirring. By using a surfactant of CMP-MO-2 named by Kanto Chemical Co Inc., they finally recovered 86% of Si with a purity of 99.999%. Similarly, Lee et al. [30] simplified the combination of extraction reagents and studied the effects of different mixing ratios of HF and HNO₃ on the purity of c-Si. A lower HF content could not completely remove the Al electrode and SiN_x, and sufficient HNO₃ was required to remove Ag. The results showed that the removal efficiency of Al, Ag, and SiN_x was the best when the ratio of HNO₃:HF was 83:17. KOH and NaOH solutions can also effectively remove Al electrodes. Klugmann-Radziemska & Ostrowski [31] first used KOH to remove the metal coating on the battery surface and then used the mixture of HF, HNO₃, and CH₃COOH for further refining. Shin et al. [32] removed Ag and Al through stepwise treatment of HNO₃ and NaOH and removed the SiN_x layer with an etching paste containing H₃PO₄, thus successfully purifying Si wafers without using HF. The multi-step treatment of acid/alkali solution effectively completes metal extraction, neutralizes each other, and reduces environmental hazards.

Hydrometallurgy is robust and mature by using chemical reagents to soak the battery to recover the Si wafer. Removing the Ag and Al electrodes is relatively simple by using simple and common acid/alkali solutions. However, as a structural ceramic material, SiN_x has a strong corrosion resistance. Studies often use HF, hot concentrated H₂SO₄, and Br₂ mixtures to remove SiN_x. However, the above chemicals have severe hazards to human health, are highly unfriendly to the environment, and are extremely dangerous to use and operate in laboratories [41].

2.5 Continuous physical and chemical processing

Since physical treatment is mainly for separating components, and chemical treatment is primarily for the purification and recovery of materials, combining the two methods can constitute a complete component recovery process.

Pagnanelli et al. [42] obtained the coarse and fine components by physical crushing. They separated EVA by heat treatment at 650 °C for 1 h and then chemically treated the fine components (0.08–0.4 mm and < 0.08 mm) with H₂SO₄ and H₂O₂. It was found that Fe, Al, and Zn mainly existed in the 0.08–0.4 mm component. In contrast, Ag, Cd, and Cu mainly existed in < 0.08 mm components, which provided a reference for subsequent targeted recovery.

Xu et al. [43] proposed a new method of coupling solvent thermal expansion and pyrolysis (SSTD) (Fig.7) to recover the complete c-Si structure. EVA is expanded by exposing the PV module to heated and pressurized organic steam, establishing a gas release channel for subsequent heat treatment. It prevents the Si wafer from breaking due to the gas accumulation and pressure inside the module. They first studied parameter optimization for mini-modules and then scaled up the experiment to commercial components. They found that the average integrity of Si wafers under this treatment method (86.11%) was much higher than that of thermal decomposition alone (9.26%). The indicators of recovered Si wafers met the needs of recycled PV modules.

Huang et al. [44] adopted a three-step process to recover crystalline Si components. First, the junction box and Al frame were removed mechanically. Then, the EVA and backplane were removed by combustion. Finally, metal elements such as Ag and Cu were recovered by chemical dissolution and electrolytic metallurgy separation. The study showed that more than 90% of the Si in the original Si wafers was recovered, meeting the specifications of solar-grade Si. The continuous physical and chemical treatment separates the components of the PV module. At the same time, a pure single component is obtained, constituting a complete recovery process.

3 Regeneration and utilization of Si in waste c-Si PV modules

The existing research mainly explores the recovery of Ag, Cu, Al, and other metal elements in PV modules, and few studies have been conducted to explore the recovery and utilization of c-Si. Therefore, exploring the recycling ways and resource utilization of crystalline Si in PV modules contributes to the sustainable development of the PV industry.

3.1 Regenerative solar cells

Si in the batteries can be extracted from ferro-Si, metallurgical-grade Si, or solargrade Si [45]. Broken and impure Si wafers cannot be reused directly for regenerative PV modules. They are often remelted through energy-intensive treatment and used again as raw materials for PV cells [46]. Therefore, the recovery of complete Si wafers has become the key to regenerative Si cells.

Lee et al. [30] found that the obtained crystalline Si can be reshaped into conventional solar cells under the optimal etching conditions (an HNO₃ to HF volume ratio of 83:17). Although its efficiency was 0.6% lower than commercial cells, it still had a good application prospect. Park et al. [47] obtained undamaged wafers through pyrolysis, then used HNO₃ and NaOH to remove Ag and Al electrodes, and mechanically removed the anti-reflecting coating, emitter layer, and p-n junction. The performance of the regenerated wafers was basically the same as that of the original industrial wafers, and the regenerated cells showed a comparable efficiency to the initial cells. The SSTD technology proposed by Xu et al. [17] can recover the complete Si cell from the PV module. At the same time, it was observed that the surface of the Si crystal had a textured structure, and the back was a smooth plane, which was very similar to the structure of the original Si crystal. Then, the Si crystals were soaked in a mixed solution containing Cu²⁺ and Ag⁺ for further purification. The thickness, resistivity, and carrier life of the treated Si crystals were similar to those of standard commercial Si crystal components, showing good performance.

3.2 Thermoelectric components

In addition to recycled batteries, Cao et al. [48] modified waste c-Si into thermoelectric modules, providing a new way to recycle PV Si waste. Under the continuous treatment of chemical reagents, the metal impurities on the surface of the battery were removed. After drying, the dopants containing P and Ge were ball-mused and sintered. The thermoelectric value of the sintering product obtained was 0.45 at 873 K, the highest among Si-based thermoelectric products. Bumba et al. [49] used ground PV waste (mass fraction of Si > 90%) to prepare a nickel (Ni) silicide catalyst, which was then used to catalyze a methanation reaction. Compared with a pure Ni catalyst, the catalytic activity of the two catalysts was similar in the first 30 min of the reaction. However, the activity of the pure Ni catalyst decreased after 30 min. The catalytic activity of Ni was thermodynamically stabilized due to the strong bonding between Ni and Si, resulting in the activity of the Ni silicide catalyst being about 20% higher than that of the pure Ni catalyst.

3.3 Electrode materials

In the era of global warming and energy shortage, efficient and clean lithium batteries (LIBs) are widely used in energy storage, and the demand for high-energy-density electrode materials has increased sharply [50]. With a theoretical capacity of 4200 mAh/g, Si anodes are considered the best energy storage material to replace graphite (372 mAh/g) [51]. Eshraghi et al. [51] used a mixture of HF and ethanol to purify Si wafers and nano-synthesized them to prepare Si anode materials. First, ball milling was used to reduce the size of Si. It was found that 250 r/min was the best for reducing the diameter of the Si particle. When evaluating the processing time ranging from 1 to 40 h, it was concluded that 10 h was the minimum time required to obtain high-performance Si anodes. The above conditions formed a network of conductive carbon particles between nano and micron particles, which was necessary to improve the performance of the battery (the battery capacity reached 1400 mAh/g). Zhang et al. [52] converted the Si wafer into porous Si (p-Si) and made Si-containing electrodes under alloying. They also reduced the Si particle size by ball milling, followed by alloying/dealloying in a LiCl-KCl molten salt bath at 500 °C to further reduce the Si particle size. Finally, they produced the p-Si structure through the volume expansion/contraction effect during the lithiation/de-lithiation process. The obtained p-Si had a specific surface area of 19.4 m²/g and a capacity retention rate of 91.5% after 200 cycles, showing a good Li storage performance. Boon Tay et al. [53] mixed the recovered PV Si with graphite to prepare anode materials. Because the large particle size Si component contained Cu solder, which affected the subsequent hydrometallurgy, they leached Si by crushing and screening the components with the smallest particle size (< 0.25 mm). Then, they adopted acid leaching treatment to remove Ag, Al, and other metal elements and recovered Si without metal residues. The recycled Si-graphite anode and commercial Si-graphite anode were then ball-milled and mixed, and an electrochemical analysis was performed. The cyclic voltammetry characteristics of the two were similar, but the performance needed to be further improved because the capacity of the recycled Si-graphite anode was only retained at 87.5% after 200 cycles.

In the above study, the pure Si wafer was selected as the material for the regenerative electrode, while Wang et al. [54] focused on the influence of different metal residues on the performance of the regenerative electrode. They ground the Si wafers into nano-Si (WSNP) by ball milling. Meanwhile, they used different chemical treatments to obtain Al-removed silicon (W-rAl) and Al-removed silver silicon (W-rAlAg), and also purchased pure Si for comparison. Electrode materials (WSNP@C, W-rAl@C, W-rAlAg@C, and Si@C) were prepared by introducing carbon nanotubes and nitrogen-doped carbon into the above four nanoparticles. It was found that the surface energy and active site of the Si@C composite were increased by the nitrogen doping layer, which improved the storage efficiency of Li⁺. It was also found that WSNPs@C, W-rAl@C, W-rAlAg@C, and Si@C can provide the initial discharge capacities of 1671.7, 2273.3, 2334.5, and 2225.5 mAh/g, respectively. The Coulomb efficiency for the above four was 54.04%, 48.41%, 44.99%, and 47.06%, respectively. The WSNP@C had a low discharge capacity due to the weak chemical bond between Al and Si. In comparison, the Coulomb efficiency of W-rAl@C at 0.5 A/g current density was as high as 99.23% because of the good electrical conductivity derived from Ag.

3.4 Other ways

In addition to recycling Si on crystalline Si cells, the reuse of Si-based trash in the PV industry has received extensive attention, including metallurgical Si slag, Si cutting waste, etc [7]. Although the Si purity of this waste is much lower than that of PV crystalline Si, it still provides a way for regenerating PV high-purity Si. Zhang et al. [55] used this waste material to manufacture a multifunctional Si₃N₄@SiO₂ nanofiber sponge. Due to its flexibility and good thermal insulation performance, it has a promising application prospect in aerospace, thermal insulation, and other fields. Liu et al. [56] then synthesized the Si-based waste into silicon carbide-aluminium nitride (SiC-AlN) ceramic composites with a high thermal conductivity. The application prospects of recycled Si will be expanded by nanosizing the Si-based trash and making composites with other materials.

4 Bottlenecks of the PV recycling industry in China

Because China sits in first place in PV production and implementation in the world, it will face the first round of recycling challenges in the next 5 to 10 years. PV module recycling has a colossal production value, but recent development status in China is unsatisfactory. At this stage, the number of decommissioning PV modules in China has yet to explode, and it does not have a considerable recovery scale [15]. According to the revenue of many waste PV module recycling enterprises, the economy of PV recycling is not ideal, mainly because of the high recycling cost and the insufficient amount of centralized recycling. If PV module recycling does not produce scale effects, it cannot create economic benefits [57]. It has led to little desire for PV module recycling in the domestic market. Under the background of the decommissioning of large volume PV in China in the future, it is necessary to fully understand the existing research, innovate, or improve the existing PV recycling process, and pursue more low-energy and low-emission technologies. Through these efforts, it will be possible to reduce the recovery cost and increase the recovery income, and stimulate the enthusiasm of the market for PV recycling, which eventually promote the sustainable development in the PV industry.

Currently, the recycling of waste PV panels is mainly concentrated in small enterprises and self-employers, who disrupt the regular economic order by bidding up the recycling price, resulting in almost no “board” to be collected by regular recycling enterprises. However, these small workshops do not conduct environmental protection treatment after recycling PV modules and only sell them after simply dismantling and removing the frame, cable, and glass. They directly incinerate or landfill the rest [58]. It not only wastes valuable metals but also creates environmental risks. PV modules either gradually leak heavy metal elements into the soil in the landfill process or release SO₂, HF, HCN, and other poisonous gases during the incineration. Once the toxic gases are inhaled by organisms, the subsequent impact on the ecological environment may be even more severe.

In addition, the waste PV module resources are seriously dispersed in China. It is difficult for formal recycling enterprises to concentrate on obtaining supplies, which is also a major bottleneck in the recovery of PV resources. At present, 40% of the total installed PV power in China are distributed PV power stations, and 60% are centralized PV power stations [59]. Among them, the centralized PV power station is mainly held by state-owned enterprises. Due to the lack of top-level design, there is no clear policy guidance in PV panel decommissioning, resulting in difficult recycling access, complex cross-regional flow, uneven recycling technology, unsmooth circulation path, and other problems [16], which affect the enthusiasm of PV recycling enterprises and the development momentum of the market.

PV module recycling in China is rudimentary. In comparison, “First Solar of the United States” has a complete product recycling mechanism for PV modules, which can ensure that all PV module products are recycled and the valuable elements are recycled [15]. In the European Union, “PV cycle” is one of the organizations that manages the recycling of solar panels, with 90% of glass from used components recycled for new products and 95% of semiconductor materials used for new solar components [60]. The PV recycling system abroad is worth learning. The future China will have a large volume of scrapped PV c-Si cells, it is worth continuing to conduct in-depth research on how to avoid wasting valuable Si (separation and recovery process) and how to create more value from the scrapped PV waste Si (recycling process). The construction of a complete recycling industry chain in China must be inseparable from the technical support in each of the above processes. Meanwhile, actively developing and improving top-level policy guidance, standardizing the recycling process and standards of waste PV modules, and implementing policy incentives and support for regular PV recycling enterprises can better meet the arrival of the “retirement tide” of PV panels.

5 Conclusions and prospect

Crystalline Si PV cell modules have a relatively complete separation and recovery process. Physical, heat treatment, and chemical treatment methods effectively targeted at different links in the recycling process. How to achieve more efficient, low energy consumption, low cost, environmentally friendly recycling, is the main direction of future research.

In addition to Ag, Al, and other metal elements, the value of Si cannot be ignored. The outer frame and junction box can be removed mechanically, the physical and heat treatment exposes the wafer cell from the sandwich structure, and the metal elements can be extracted and recovered by chemical reagents. At the same time, this process also promotes the purification of crystalline Si. It should be noted that simple heat treatment will lead to expansion when the encapsulation agent EVA decomposes. The decomposition gas has no escape channel between the tight sandwich structure, resulting in varying degrees of damage to the obtained Si crystals, which often cannot be applied to the regeneration of the battery. As far as chemical treatment is concerned, the easy operation of acid leaching and alkali leaching can meet the requirements of metal recovery and crystal Si purification. However, the subsequent waste liquid with environmental hazards still needs to be treated appropriately. Waste PV module crystalline Si has a higher Si purity and a good crystal structure than PV Si-based waste, and it is more likely to be reused in high-purity Si demand industries. At present, the research on the regeneration of waste solar crystalline Si has gradually been paid attention to. The recycling of waste crystalline Si is not limited to solar cells based on the plasticity of Si, and many scholars have begun to explore other application ways. For example, since Si has a much higher theoretical cell capacity than graphite, using methods such as ball milling to manufacture porous structures or doping, the prepared Si anode materials also show excellent electrochemical properties under certain conditions. Among them, the nanosizing Si is the key to ensuring a good battery cycle life, and the size of Si is the decisive factor for the electrochemical performance of Si-based anodes. At the same time, the incomplete removal of Al and Ag in crystalline Si solar cells will affect the nano-crystallization of Si and the performance of the recycled Si anode. Of course, good electric heating devices can be prepared using Si-based trash through element doping. Paying attention to the plasticity of Si, expanding its properties by means of porosity or doping, and seeking its use in other fields are possible pathways in future research.

Regarding the top-level design of PV module recycling, there still needs to be unified industrial recycling standards and technical routes in China and abroad. The gradually emerging complete life cycle evaluation provides a realistic and practical data basis for industry norms and policy formulation. Although many scholars have used life cycle assessment to conduct many studies on the energy recovery period, environmental impact, and carbon emissions of PV modules, these studies mainly focus on the production stage of PV modules but on the use and disposal stage. In addition, previous studies were primarily based on hypothesis theories (such as recovery rate hypothesis, measurement growth hypothesis, and experimental data hypothesis), lacking data support for actual disposal paths [61]. Using different recycling treatment paths for the resource utilization of crystalline Si cells will significantly enrich the data samples for the life cycle assessment of PV resources. It can provide sufficient scientific support for the complete life cycle management of the PV industry chain and carbon footprint accounting, and also helps point out the direction for the future development of the PV recycling industry.

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